A tangential issue:
Doesn’t all this negate the entire premise of the SETI project?
Assuming a broadcast of “normal” broadcast power, the inverse square law would suggest that by the time a radio signal reached earth, even from the nearest star, it would be so weak it would be undetectable.
So why the SETI project?
I believe SETI is looking for signals stronger than normal earth radio signals, strong signals aimed outward directly at us, either intentionally or by coincidence
The Chinese are currently building a radio telescope larger than Arecibo and it’s supposed to be more steerable. I don’t know by how much, though. Its scheduled completion is next year, but I suspect that that’s optimistic. It’s called the Five hundred metre Aperture Spherical Telescope, if you want to google it.
Both Arecibo and the full 500 meter telescope are steerable to a very small extent - the reciving antennae at the focus can be moved a small distance, allowing the telescope to track an object, or scan a small region, for a short length of time. A far cry from a fully steerable telescope. The 500 metre telescope is being designed with deformable reflector components, and it seems that this will allow it to steer an effective subset of its receiving area - so it can act as a very big telescope in much the same manner as Arecibo, or with much greater flexibility in pointing, albeit with a reduced area - although still an area about the same as Arecibo. Very cool. Very complex. One suspects getting the bugs out will take longer than they hope.
Yes, but a solar orbiting telescope would be able to cover nearly the entire sky (except for in the direction of the Sun relative to the orbital position). An orbiting array could be adjusted for optimal configuration for a given bandwidth. Both would be at least an order of magnitude more caapble than any ground based observatory. Unfortunately, with that capabiltiy comes cost, which is probably also an order of magnitude or more to deploy such a system, and of course the substantial difficulty in maintaining a system beyond Earth orbit, but there is no way to developed comparable capability in an terrestrial-based observatory.
Stranger
Incidentally the figure linked by the OP looks wrong. If the galaxy is 100,000 lightyears across and the radius of our radio signals is 92 years, then the diameter of the small circle should be about 1/500th of the width of the picture, while it looks like its more than 1/100th.
Arecibo also has movable panels around the edge of the dish that can compensate a little for movement of the focal instruments, allowing a bit more aiming.
Except one…
The Morimoto - Hirabayashi Message to Altair (sent in 1985) arrived in 1999, and we should be getting our reply any moment now - assuming there is someone up there to reply (the Krell, perhaps).
OK. So my quick mental calculations (using the inverse square law) suggest that for that to happen, the transmitter would have to have a power output greater than our sun.
A transmitter of such magnitude would be pretty obvious to us long before we became interested in their transmissions.
I find it difficult to imagine that any civilization that is capable of building such a transmitter would waste its energy sending “hello world” messages to us. More likely, they would just trundle over here and land on the White House lawn.
This is the exact reason that Nikolai Kardashev developed his Kardashev Scale. He wanted to calculate how far away a broadcast signal could be detected, assuming that the civilisation sending the signal had access to various increasing amounts of power.
Assuming that a civilisation has access to similar amounts of power to that available on our current Earth, the broadcast capabilities would be pretty limited. So he considered also the case of a civilisation that could transmit with the power of a star, and with the power of an entire galaxy (much better, if a little impractical).
http://adsabs.harvard.edu/full/1964SvA.....8..217K
In practice, a sensible civilisation wouldn’t broadcast its signals, but would focus them on certain intended targets. Much less chance of accidentally alerting the interstellar raptors.
What would an advanced civilization make of AM radio? It would be so primitive that they likely would ignore it. Others have speculated that aliens would use a “natural” frequency (like the hydrogen frequency); that assumes that alien races would be actively looking. i don’t see anything coming from this.
One of the problems with the Kardashev scale is a gross energy utilization metric that assumes that a more advanced alien species will use larger amounts of energy, up to a galactic scale. Although application of this scale does very roughly follow the development of technology in our industrial development, it doesn’t necessarily follow that technological development will require monotonic escalation of global energy usage. Practically, a better metric might be efficiency of data storage or precision of control of fundamental forces.
The 21 cm line resulting from the [THREAD=612372]hyperfine transition of neutral hydrogen[/THREAD] is not a frequency you would use to transmit signals across interstellar space because it is the most common naturally occurring radiated frequency. Deliberate communication would most be narrow beam transmission in an X-ray frequency band. However, any signal of sufficient strength provides a steady or sinusoidal signal frequency with a constant range of amplitudes over a narrow frequency band (e.g. any artificial signals) could potentially be detectable by a sufficiently sensitive receiver.
Stranger
How far could a signal is sent from Earth Orbit go to? Lets say a purpose built transmitter in High or Medium Earth orbit?
What size? How much power throughput? What frequency band range, data rate, and bandwidth? The question is too open-ended to offer a definitive answer.
There is no limit to the range that a signal could be transmitted up to the point the metric expansion of space will achieve a distance defined by the null interval (the distance in space-time at which a photon would take infinite time to reach). We could potentially bound the problem using conventional technology by assuming a transmitter of power no greater than some specified solar collection area times the Sun’s luminous flux at Earth orbit times the throughput efficiency of a gyrotron or free electron laser. Understand that transmitting a useable carrier signal across distances to even the nearest stars that were powerful enough to be received by a single reasonably-sized receiver would be on the order of gigawatts or greater to transmit video or high rate data signals comparable to broadcast television, and that is assuming a very narrowband carrier. Here is an interesting paper that was published in International Journal of Astrobiology that doesn’t answer your question but provides enough details for further consideration.
Stranger
I thought our experience (n=1) implies that as a civilization advances, it tends to “leak” less (eg using cable and fibre optics or more directed transmissions). Also, I’m given to understand that perfectly compressed data transmissions look like noise to an outsider.
That implies that developing civilizations might get loud, then quiet down. Maybe they get loud again as they go interplanetary.
Note that Kardashev himself was only trying to imagine the extreme cases of transmission strengths; most of the hype about KI/II/II civilisations has come from people who have expanded on his ideas.
Nice thread. Carry on.
Perfectly compressed data may be indistinguishable from noise, but nobody actually wants perfectly compressed data, for exactly that reason. If one or two bits get flipped due to actual noise, you could end up not even knowing the difference, and de-compressing out a completely different, but still intelligible, message. In practice, after your compression has removed all of the redundancy, you want to systematically put a little bit of redundancy back in, so you can detect or even correct some number of errors. And there are a few fairly obvious ways to put this systematic redundancy in, and few good reasons not to use the obvious methods, so actual communications are likely to have a recognizable signature if you look for it.
Based on our ongoing experience, I would say that the period during which a civilization uses radio is fairly short. We were fairly loud in radio frequencies in the latter half of the twentieth century but that’s changing. I would expect in 100 years that we’ll probably not be using radio very much. It’s been said before, but expecting alien civilizations to be broadcasting in radio is like primitive people trying to detect more advanced civilizations by listening for really loud talking drums. Lasers are more likely but who knows? Lasers may also be short lived too. Really advanced civilizations may be using gravitational waves, or something we haven’t even yet conceived of.
–Mark